Aspects of the present invention are directed to non-thermal methods of machining a profile pattern and, more particularly, to methods of machining a profile pattern in a ceramic coating without an exertion of lateral force or causing thermally induced stresses.
One such application of a ceramic coating which can benefit from a profile pattern is a turbine shroud. The turbine shroud is used in gas turbines to form the circumferential perimeter of the gas path above the turbine buckets. Turbine shrouds are often formed with a ceramic coating, which is frequently referred to as a thermal barrier coating (TBC), such as plasma sprayed Yttria stabilized zirconia, YSZ and a MCrAlY bond coat on a superalloy substrate, where M can be Nickel, Cobalt, or Iron.
Tight clearances between the bucket tip and the shroud flowpath are desired to minimize gas leakage over the tip and hence improve turbine performance. It is often difficult, however, to run the turbine with tight clearances because a circularity of the casing is not maintained throughout all phases of the turbine cycle and, especially, during thermal transients. For example, centrifugal loads as well as differences in thermal responses between the turbine bucket and the turbine shroud around the circumference of the turbine may lead to non-rounded expansion of the turbine casing. Here, while the ceramic coating provides for thermal insulation of the underlying metallic substrate, the ceramic coating is harder than the bucket tip and can damage the tip during a rubbing occurrence.
One solution to reducing clearances and allowing turbine bucket-shroud rubbing is to have an abradable coating as the innermost surface of the turbine shroud. In this case, the ceramic coating is sprayed thereon in patterns, such as curvilinear patterns, w-shaped patterns, or “waffle” like patterns. The patterns in the ceramic coatings are employed to aid in the abradability of the ceramic coatings. This prevents damage to the turbine buckets that would otherwise occur as a result of the turbine buckets rotating within the turbine shrouds and cutting broad swaths of material away from the ceramic coatings. Another important feature of the patterns is to direct airflow in the turbine during operations thereof. This improved directionality of the airflow above the blade tip improves turbine performance.
Currently, the patterns are formed by utilizing shielding masks during applications of successive coating layers. In some applications, the ceramic coating of increased porosity is applied onto the surface of a conventional TBC while in other applications a more porous coating is applied directly onto the MCrAlY bond coat. A particular pattern can be used in either of these cases to improve abradability and aerodynamic performance in the turbine.